The determination of flow stress of tubular material for hydroforming applications

Tube hydroforming (THF) process is nowadays a developed and successful way for forming of complex shapes with less operations required compared to conventional tube forming processes. FE simulations are powerful tools which allow a remarkable saving in time and money when developing a feasibility study or a prototype phase for a new THF operation. However the successful use of FE simulations depends on several factors such as interface friction or material properties. While in the past flow stress used as input in FE simulations was generally obtained from tensile test conducted on the sheet prior to rolling and welding operations in case of rolled and welded tubes, or tensile test conducted on the whole tube for extruded or seamless tubes, in the last years tube bulge test has been widely used. In this research test experiments, coupled with suitable analytical model of the process, are used to identify the flow stress of tubes (under biaxial stress state) by measuring geometrical features of the tube under study as the fluid pressure increases. In the present paper a new approach to tube bulge test is described. The innovative aspect is related to the fact that the tube ends are blocked and so the equilibrium expression in axial direction normally used to calculate stresses is no more valid. The stress state is therefore derived from the flow rule and the volume constancy. The new proposed analytical model was validated by means of FE simulations. Results of test conducted on seamless tubes show that bulge test allows to obtain material properties for high strain thus avoiding possible errors in extrapolating flow stress for FE simulations.